1. Field of the Invention
The present invention relates to a method of producing a high-strength steel sheet which exhibits good workability and which can be formed into a can having high strength by drawing with minimized earing.
According to JIS G 3303, the temper degrees of black plate used for can making are classified in classes T-1 to T-6 in order of softness. A target value of Rockwell hardness (HR 30T) is determined for each of the temper degrees. Sheet materials having a temper degree of T-3 or less are referred to as "soft materials", and sheet materials having a temper degree of T-4 to T-6 are referred to as "hard materials".
Soft steels having a temper degree of T-1 or T-2 are generally used as materials for making two-piece cans such as drawn and ironed cans, drawn and redrawn cans, drawn and thin redrawn cans and the like, because of their property of deep drawability. In recent years, however, the steel has more often been thinned in order to decrease the cost of the can. Since the strength of a steel sheet must thus be increased to compensate for the thickness decrease, hard steels having a temper degree of T-4 to T-6, which is higher than that of conventional can materials, are more often used. Higher strength is also required for three-piece cans as thinning of cans progresses. These are generally produced from a material having a temper degree of about T-4 to T-5.
Particularly when two-piece cans are produced, if significant earing is encountered during drawing, the yield of product deteriorates, and difficulties such as breakage of earing and the like occur during can making, thereby decreasing production efficiency. For this reason, a steel sheet is required which encounters reduced earing during drawing. In addition, when the height of a can is achieved by drawing, as in drawn and redrawn (DRD) cans and drawn and thinned redrawn (DTR) cans, good drawability is also required of the steel.
However, although many conventional steels have high strength they exhibit poor drawability and undergo excessive earing during can making. There is accordingly a serious demand for improving the characteristics of these can-making materials.
2. Description of the Related Art
Examples of methods of producing black plate of a hard material having good workability and a temper degree of T-4 to T-6 include the methods disclosed in Japanese Patent Laid-Open Nos. 58-27931 and 2-118027.
Japanese Patent Laid-Open No. 58-27931 relates to a method of producing a base steel sheet for tin or tin-free steel sheet. This method comprises hot rolling low-carbon aluminum killed steel containing 0.01 to 0.04 wt % carbon, pickling, cold rolling, annealing, and temper rolling. However, it is difficult by this method to obtain a steel sheet which satisfies the level of workability required for thin material for forming into two-piece cans.
Japanese Patent Laid-Open No.2-118027 discloses a method of producing a steel sheet for making cans. This method comprises hot rolling a continuously cast slab consisting of 0.004 wt % or less carbon, 0.05 to 0.2 wt % aluminum, 0.003 wt % or less nitrogen and 0.01 wt % or less niobium, cold rolling the resulting material at a reduction of 85 to 90%, continuously annealing the material and temper-rolling the material at a reduction of 15 to 45%. The steel sheet obtained by this method exhibits excellent deep drawability, and encounters reduced earing during drawing. However, the method presents a problem with respect to the low level of work hardening during can making, which is due to a small amount of strain aging.
In order to secure the strength of the body of a two-piece can such as a DI can, a DRD can, a DTR can or the like, work hardening comprising drawing, ironing and the like is employed during can making. The use of steel sheets obtained by the above methods and exhibiting less strain aging thus prevents the production of a can having a body with sufficiently high strength.